fix(logtail): keep mail.log in /data and reconcile stuck rows (v1.x closure phase 2)

Move the delivery log from the ephemeral /var/log to /data/log/mail.log so
the lines that resolve a queued send-log row survive a container recreate.
postlogd writes it as postfix, the panel reads it through the selfpost group
(dir 2750, file 0640, normalised every start); backups exclude log/.

Close the residual gap with a queue sweep: rows queued for over two minutes
whose id postqueue -p no longer lists are marked bounced. The sweep waits
until the tailer has read the log to its end and does nothing when the queue
cannot be listed, so a message in flight is never touched.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Co-authored-by: Cursor <cursoragent@cursor.com>

README.md

@@ -52,7 +52,8 @@ missing PTR record for you. Details: [Operator guide](docs/guide.md).
 - Docker + Compose v2 on the host
 - A reverse proxy in front of the panel (SelfPost never terminates HTTPS itself)
 - Rough sizing: **1 vCPU**, **512 MB–1 GB RAM**, **8–10 GB disk** (send log and
-  rotated `mail.log` are the main growth drivers)
+  rotated `mail.log` are the main growth drivers; both sit in the `./data`
+  volume, and both are capped — 90 days and 14 files by default)
 
 ### Network and IP